Volume histogram

Hi,
In the volume histogram, in MT4:

1. What's the unit? (Volume = 8, 12, 42)

2. How is this measured? In stock it seems fairly easy but in Forex? 10 million brokers moving money in nanoseconds, no central computer... sounds strange.

There is no Volume in Forex. -> OTC Market
The Volume you see on MT4 is more a number of how many ticks you've got int that candle. You could aswell just write a random number on your volume bar.
Completely useless.